1. Field of the Invention
The present invention relates to a magnetic recording medium of metal thin film type and more particularly, a magnetic recording medium capable of optimizing the property of a protective layer which is developed over a magnetic recording layer for improvement in the practical use and a method of producing the same.
2. Description of the Prior Art
A magnetic recording medium of ferromagnetic metal has been known which is formed by developing a layer of metal alloy containing Co, Ni, Fe, or their combination on a substrate of non-magnetic material, e.g. polyester film, polyimid film, or other polymer film, by a conventional method such as vacuum vapor deposition, sputtering, ion plating, or the like. Such a known magnetic recording medium is capable of increasing the recording density as compared with a coating-type magnetic recording medium. For ensuring high density recording, it is essential that operational error during recording and reproducing is minimized, spacing loss between magnetic head and recording medium is eliminated, and practical durability is increased. However, the metal thin film type magnetic recording medium has a layer of thin film of metal material which is too thin to provide a desired degree of durability. Particularly, the thin film metal is low in the resistance to corrosion and will easily be affected. For improvement, techniques have been proposed in which the thin film of metal material is coated with a protective layer which is in turn covered with a lubricant layer as a top coating. Also, an improved method has been invented by us (as described in U.S. patent application Ser. No. 521,987 filed on May 11, 1990) in which prior to development of a protective layer, impurities including occluded water are removed in the form of an out gas from the thin film of ferromagnetic metal material by heating so that the bonding strength between the ferromagnetic metal thin film and the protective layer can be increased.
Unfortunately, the out gas removed from the magnetic recording medium tends to pollute the surface of the protective layer, thus interrupting the coupling between the protective layer and the lubricant layer. This will result in head clogging during use of the magnetic recording medium. Also, moisture released from a non-magnetic substrate of the magnetic recording medium is trapped between the ferromagnetic metal thin film and the protective layer at a stage of forming the protective layer, reducing the bonding strength inbetween. As the result, the still frame life (or resistance to corrosion) of the finished magnetic recording medium after storage under high-temperature and high-moisture conditions will be declined. This is disadvantageous to the quality of the magnetic recording medium. It was found that the hydroxyl group (OH radical) existing at the interface between the ferromagnetic metal thin film and the protective layer exhibited an atomic ratio of more than 0.2 to a primary component metal element contained in the ferromagnetic metal thin film.